This invention pertains to nozzle flapper valves, and more particularly, to a method to stabilize nozzle flapper valves from generating a buzz at high frequencies and pressures.
Flapper valves are used in a wide range of applications and can be made in a number of configurations. Flapper valves are commonly classified by the number of nozzles (dual nozzle vs. single nozzle) and the number of fluid ports (3-way vs. 4-way). A 3-way device will have three ports: supply pressure, drain pressure, and output pressure. The output pressure is commonly called the xe2x80x9cservo pressurexe2x80x9d because it is often used to move a servo piston. Likewise, a 4-way device will have four ports: supply pressure, drain pressure, and two servo pressures. In this case the two servo pressures work in push-pull mode, where one goes up and the other goes down, allowing them to be used on opposite sides of a servo piston.
Flapper valves are also called xe2x80x9cbleed valvesxe2x80x9d because one of their key characteristics is a continuous bleed of fluid flow from a high pressure source to a low pressure drain. In its most basic form, a flapper valve consists of at least two flow restrictions, at least one of which is variable, which bleed flow from a high pressure source to a low pressure drain in such a way as to create a variable output (servo) flow/pressure which may be modulated by changing the size of the variable restriction. The variable flow restriction is typically mechanized as a nozzle that is pointed at and almost touches a movable flat surface (the xe2x80x9cflapperxe2x80x9d), although any number of other schemes is possible. The gap between the nozzle end and the flapper is typically quite small, nominally on the order of {fraction (1/10)} to {fraction (1/20)} the nozzle diameter. The variable flow restriction area then in the shape of a thin xe2x80x9ccurtainxe2x80x9d around the end of the nozzle gap. There are numerous mechanisms for changing the size of the variable flow restriction, i.e. ways of moving the flapper. Many of these mechanisms involve a mechanical assembly that moves linearly or about a pivot against a centering spring rate. Typically some means of applying an external force to the flapper assembly is provided, such as a torque motor. A torque motor consists of one or more electrical coils and a magnet and armature assembly with magnetically charged air gaps. When electrical current flows in the coils, the magnetic field in the air gaps is altered in such a way as to apply a torque or force to the flapper assembly and cause it to move. Sometimes there is a feedback spring attached to the flapper, which provides a feedback force from servo position or second stage position. (A second stage valve is used in those applications where the flapper has insufficient flow capacity to directly operate the servo.) Thus a force balance determines the position of the flapper. But the configuration of a flapper valve is normally such that the flow and pressure forces acting upon it by the fluid flow are not balanced. As the flapper moves, the flow and pressure through it change, changing the flow and pressure forces on the flapper. These flow and pressure forces can be such as to promote oscillations and instability of the flapper.
One application where flapper valves are used is high response, multistage servovalves. The first stage of the servovalve is a double acting nozzle flapper valve with a torque-motor actuated flapper and the second stage is a spool valve. The torque-motor is spring centered to null position. At the null position, the flapper is centered between the two nozzles and the nozzle pressure forces are balanced. Each nozzle is fed from a high pressure fluid or pneumatic source through an orifice. When the current through the torque-motor coil is increased from null, the resulting increase in the electromagnetic force causes the flapper to move. The flapper closes one of the nozzles and diverts flow to a spool end. The spool moves and opens one of the control ports to supply and opens the other port to return. A feedback spring provides a feedback force from the second stage position back to the flapper. The spool stops at a position where the feedback spring torque equals the torque due to the coil current (i.e., the input current). This results in the spool position being proportional to input current. In a constant pressure system, the flow to the load is proportional to the input current.
The flapper valve and associated torque motor parts that move with it represent a mass that moves about a pivot against a spring rate. This mass-spring combination has a natural frequency at which it tends to oscillate. The damping on this mass-spring combination is normally quite low. A recurring design problem with flapper valves, particularly high response, multistage servovalves, is avoiding flapper oscillation at the natural frequency. The natural frequency typically ranges from a few hundred up to around a thousand cycles per second. The flapper oscillation, which may generate an audible buzzing sound, is highly undesirable for several reasons. First, it may cause premature failure from metal fatigue from the induced cyclic stress. Second, it may cause performance problems. During oscillation, the steady state output flow and pressure characteristics will shift due to the nonlinear nature of the turbulent flow through restrictions in the flow path. This oscillation is particularly detrimental when the oscillation comes and goes, causing the output pressure and flow to shift or step in value. The oscillation may be self sustaining in extreme cases, or, in milder cases, may manifest itself as a xe2x80x9cringingxe2x80x9d or xe2x80x9cresonancexe2x80x9d in response to external inputs. For example, mechanical vibration at the natural frequency may cause the valve to buzz. It may manifest itself as a xe2x80x9cringingxe2x80x9d of the flapper position after a step current input where the flapper will oscillate with decaying amplitude before settling out. Such behavior is undesirable in high response systems. The tendency to oscillate becomes greater with increasing supply pressure. The reason is the higher the supply pressure, the higher the flow and pressure gain of the flapper. As is well known in the art of control theory, raising gains within a system usually has the effect of making it faster, but degrading its stability.
Industry has developed a number of strategies for eliminating or reducing the tendency of flapper valves to buzz. One example is addition of a damping fluid to the torque motor assembly cavity. The flapper and torque motor armature oscillation displaces a highly viscous fluid, which dissipates energy and improves stability. This technique has several disadvantages. One disadvantage is that viscous fluid is temperature sensitive since fluid viscosity varies widely with temperature. Another disadvantage is the technique is not very robust because an operating fluid leak may wash away the damping fluid during the service life of the unit.
Another strategy to improve damping is to add a shorted damping coil to the torque motor. This strategy has the disadvantages of adding expense and taking up space and results in reduced performance of the operating coils. Still another prior art strategy is to add a series flow restriction, normally downstream of the flapper valve. This reduces the flapper valve gain and improves stability, but it also degrades the steady state performance of the system. A further method is to sharpen the edges at the end of the nozzle throat to reduce the lip area that the flowing fluid pressure acts upon. This method has limited effectiveness since it does not affect the area within the nozzle on which the pressure can work. Another prior art method is to reduce the nozzle diameter and increase the nozzle gap, which reduce the pressure and flow gain. While this improves stability, it also degrades steady state performance.
The invention provides a way to stabilize a flapper valve that is very robust, very inexpensive, and that does not degrade the steady state performance of the unit. An inertia tube is added to the flow path of the flapper valve nozzle. The inertia tube has a length to area ratio of greater than 1000 in/in2.
The addition of an inertia tube to the nozzle makes the fixed size orifice of the nozzle behave like an orifice having a size that is a function of flow frequency. The inertia tube may be a straight tube, a coiled tube, a thread passage and the like.